The invention relates to a method for carrying out maintenance work on structural parts within a refrigerator housing having two parts separable from each other in such a manner that the interior space of the refrigerator housing becomes accessible, whereby the housing is opened, maintenance work is carried out and the housing is closed and finally swept with a working gas. In addition, the invention relates to a device and a refrigerator for carrying out the method of the invention.
Refrigerators are low temperature cold machines, wherein thermodynamic circulation processes occur. A one-stage refrigerator comprises essentially a working space with a displacer. The working space is alternatingly connected in a specific way to a high pressure gas source and to a low pressure gas source, so that during the forced reciprocating motion of the displacer the thermodynamic circulation processes occur. Here, the working gas is led into a closed circulation system, the consequence of which is that from a given region of the working chamber heat is withdrawn. Employing two-stage refrigerators of this type with helium as the working gas, temperatures less than 10.degree. K. can be achieved.
Generally, repair and maintenance work on structural parts located within the refrigerator housing is carried out by switching the refrigerator off and waiting for the cold regions of the refrigerator to reach room temperature. This waiting period was necessary in the absence of heating, to avoid the collection of condensable gases on the cold surfaces upon opening the refrigerator housing the condensation of which would endanger the operation of the refrigerator upon restarting. Since the temperatures of the cold surfaces of a refrigerator operated with helium are only a few degrees Kelvin (60.degree. to 80.degree. K. in a one-stage refrigerator or at the first cold stage of a two-stage refrigerator) nearly all gases present in the atmosphere must be considered condensable gases and, therefore, undesirable. Alternatively, the housing is sufficient preheated, prior to opening. The housing is then opened and the requisite work is carried out. The housing is then closed and thoroughly swept with the working gas before the refrigerator is again returned to service.
Maintenance work carried out in this way takes a long time, considering in total the actual repair work time, the housing preheat time and the restart time. Furthermore, operation of the equipment or instrument, which is cooled by the refrigerator, is interrupted during this time. With instruments having a high fill level of liquid helium (100:1 and more) heating the refrigerator for the sole purpose of maintenance is, for economic reasons alone, out of the question because it presupposes removal of the liquid helium. If, for example, a refrigerator cools the magnets of a nuclear spin tomograph, considering the instrument being inoperable for two to three weeks and the additional cost of helium, extraordinary expenses totaling tens of thousands of dollars would have to be acceptable if the maintenance work is to be carried out in the conventional manner.
In order to avoid long operating interruptions, it is known to undertake the maintenance work within a glove box. Using the glove box permits carrying out this work in a protective gas atmosphere. In this way, for example, condensable gases which would penetrate into the working or cylinder space of the displacer and condense on the cold surfaces are avoided. Using the glove box, therefore, has the advantage that maintenance work can be performed at still low temperatures without the requirement of preheating the cold areas of the refrigerator and the connected equipment or instrument.
Using the relatively expensive glove box is possible however, only where sufficient space is available. Moreover, the length of time for performing the maintenance work is still somewhat excessive (at least 2 hours). One reason for this is the need for several sweeping processes with concomittant relatively high protective gas consumption to create a sufficiently pure protective gas atmosphere within the glove box. Another reason can be found in the high degree of difficulty inherent in handling tools with the gloves of the glove box. Moreover, the danger exists that, for example, a displacer to be exchanged touches the box sides or the gloves in the glove box with its cold side upon being pulled out. The consequence of this is the destruction of parts customarily consisting of synthetic foil and with that a contamination of the protective gas atmosphere. In addition, temperature increases occurring during the maintenance work, which, due to using the glove box for a relatively long time, are not neglible, so that the time spent on returning to operating temperature service is significant. Finally, the costs incurred in connection with the high protective gas consumption are not inconsiderable, particularly if helium must be used.
Another displacer exchange method has been suggested, in which a protective gas stream is maintained from the moment the housing is opened until closing to prevent undesirable gases from penetrating into the cylinder chamber of the displacer. However, in this exchange method, the possibility exists that condensable gases will reach the displacer chamber, particularly, at the moment when the displacer to be exchanged is removed from the refrigerator housing. If this is done too rapidly, then not only protective gas but air also enters the displacer chamber and condenses immediately on the cold interior walls of the housing. The condensation hinders the installation of the new displacer and can only be removed at great technical expense.